1. Field of the Invention
The present invention relates to any positioning system that uses synchronised signals from a number of reference stations. The position of the reference stations is accurately known.
2. Related Background Art
A mobile terminal receives signals from a number of reference stations, measures the difference in time of arrival between the signals and computes an estimate of its own position relative to the reference stations. In order to form an estimate in three-dimensional space a minimum of four reference station signals must be received.
A ‘dual’ arrangement is also well known where each mobile terminal transmits a signal to the reference stations that are mutually time synchronised as before. The reference stations are connected to a central control point where the position calculation is performed. The estimate of position is then transmitted to the mobile terminal. In order to form an estimate in three-dimensional space a minimum of four reference stations must receive the signal from the mobile terminal.
Factors influencing the accuracy of the position estimate, whichever arrangement is employed, are:    1. Geometry—the direct path to some reference stations may be obscured by natural or man-made objects and/or the mobile can be in a location where trigonometric calculations can give rise to large errors.    2. Multi-path—signals from the reference stations may be subject to reflection off an object (either stationary or moving) and these indirect or multi-path signals give rise to an error in the position estimate because they distort the reception of the direct signal.    3. Interference—man-made interference from other systems can block reception of the desired signal because they have much higher power levels or at lower levels of interference can cause errors in the mobile receiver signal processing.
The GPS system is the best known example of such a positioning system. In this case satellites carry the synchronised reference stations. One of the major deficiencies with GPS is that coverage in buildings and in dense urban environments is not reliable. The reason being the obscuration and attenuation of the direct path signals by buildings. Often in areas where signals may be received they are degraded by complex multi-path components.
This invention supplements GPS in these environments and can be used where GPS signals are available but greater precision is required e.g. in aircraft landing.
Spread-spectrum techniques have been developed to give improvements in a number of instances:                To enable unambiguous ranging estimates        To reduce degradation due to multi-path        To reduce susceptibility to interfering signals        To reduce detection by unauthorised users        To reduce interference to other systems        
Often, a number of these improvements can be obtained at the same time. In general, the performance of each of the above is determined directly by the bandwidth and coherence of the spread spectrum signal.
These techniques are in common use in navigation, communications and radar systems.
This invention describes a means of producing and receiving a controllable ultra-wideband (UWB) spread spectrum signal that has characteristics that are well matched to the application of precise positioning in environments where GPS is unreliable.
The nature of the signal also makes it suitable for the other named applications above.
There are two general methods of generating a spread spectrum signal. The first is by direct modulation of a carrier by a wideband signal and the second is by hopping the carrier frequency.
In the direct method, the carrier is directly modulated by a deterministic time-limited wideband waveform (spreading waveform). This spreading waveform is made to be periodic by repeating an exact replica after a suitable time interval (waveform period). For the resulting spread spectrum signal to be coherent, the carrier frequency and the waveform period must be harmonically related.
A common means is to use a pseudo random binary sequence (PRBS) as the spreading waveform. In this case to ensure coherence the carrier frequency and the clock (chipping) rate of the spreading waveform are in a harmonic relationship—this in turn ensures that the time period of the PRBS is harmonically related to the carrier frequency. Another means of producing a coherent spread spectrum signal is to modulate a carrier signal with a chirp waveform that is repeated at a rate that is harmonically related to the carrier frequency. To send messages, the spreading waveform is first modulated with the message signal. The resulting waveform then modulates a carrier.
In the PRBS example, the chipping rate and the code length of the PRBS signal must be high enough to spread the signal energy over the whole of the desired bandwidth. Each transmission has a unique spreading waveform by which it can be recognised and retrieved and accordingly a receiving station needs to be able to generate the same spreading waveform in synchronism with the transmission in order to recognise and recover the message.
A multiple access system can be devised by using unique spreading waveforms. All other transmissions, which are combined with different spreading waveforms, appear as noise in the receiver. If the direct modulation is by PRBS the system is termed code division multiple access (CDMA).
In the frequency hopping method, the frequency of the transmission is changed as a function of a predetermined PRBS. Frequency hopping is usually limited to relatively slow hopping rates (typically a few kilohertz) because of inherent limitations in the equipment used.